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Abstract

A new kind of plenoptic imaging system based on Laser Optical Feedback Imaging (LOFI) is presented and is compared to another previously existing device based on microlens array. Improved photometric performances, resolution and depth of field are obtained at the price of a slow point by point scanning. Main properties of plenoptic microscopes such as numerical refocusing on any curved surface or aberrations compensation are both theoretically and experimentally demonstrated with a LOFI-based device.

Figures (4)

Experimental setup of the synthetic aperture LOFI-based imaging system. The laser is a 10 mW cw Nd:YVO4 microchip collimated by lens L1. A beam splitter sends 10% output power on a photodiode connected to a lock-in amplifier which gives access to the amplitude and phase of the signal. The frequency shifter is made of two acousto-optic modulators which diffract respectively in orders 1 and −1 and give a round trip total frequency shift of Fe = 3 MHz. x-y plane is scanned by galvanometric mirrors MX (scan in the x direction) and MY (scan in the y direction) conjugated by a telescope made by two identical lenses L3. f3 is the focal lengths of L3. αX and αY are the angular positions of MX and MY. L4 is the objective lens, f4 is its focal distance and r is the final waist of the laser after L4.

Images of 40 μm diameter silica beads. Observation (a) with classical bright field microscope; (b), (c), (e), (f) with LOFI-based microscope. Image (b) is a raw defocused image (L = 300 μm resulting in a spot size of 100 μm in the target plane) with highly aberrant lens (NA = 0.25). From (b), image (c) is obtained after numerical refocusing and (d) with both refocusing and numerical aberration compensation (expected resolution of r / √2 ≈1 μm). On (e) comparison with a numerically refocused image acquired with an aberration-free commercial objective (NA = 0.15, r / √2 ≈1.4 μm). (f) is the PSF used for filtering which reflects the aberrations which are compensated. Images (b)-(f) have a size of 256*256 pixels. Images from Figs. 3(a), 3(b)-(d) and 3(e) have not been acquired on the same zone of the field: this explains why no correlations on the placement of beads can be observed. Color map of images (b) to (f) have no physical significance and thus are not displayed; false color are automatically scaled between the minimum and the maximum of each image’s amplitude.